1. Field of the Invention
The present invention relates to a method for backscatter interrogator reception as well as to an interrogator for use in a modulated backscatter system (MBS).
2. Description of the Related Art
Modulated backscatter systems are known in the art and are the basis for the recently developed so-called radio frequency identification (RFID) systems. These RFID systems are for example used for identifying and/or tracking equipment, inventory, persons or animals. RFID systems are radio communication systems that allow communication between a radio transceiver, called an interrogator, and at least one so-called tag. In an RFID system the interrogator communicates with the tag by transmitting an unmodulated radio signal which is received, modulated and transmitted back by the tag to the interrogator.
The general idea underlying a modulated backscatter system is shown in FIG. 1. Similar to the field of radar technology, backscatter communication is based on the idea that electromagnetic waves are reflected by an object. The efficiency with which an object reflects electromagnetic waves is described by its radar cross-section. Objects that are in resonance with the wave front that hits them, as it is the case for antennas at the appropriate frequency, for example, have a particularly large radar cross-section.
In the general modulated backscatter system shown in FIG. 1, a signal P1 is emitted from a transmitting circuit 101 of interrogator 100, a small portion P1′ of which reaches the antenna 111 of the tag 110.
A proportion of the incoming power P1′ is reflected by the antenna 111 of the tag 110 and returned as power P2. The reflection characteristics of the antenna 111 can be influenced by altering the load connected to the antenna 111. In order to transmit data from the tag 111 to the interrogator 110, two different load impedance values Z and Z′ in the embodiment of FIG. 1 are alternately connected with the antenna 111 in time with the data stream to be transmitted. In this way, the amplitude of the power P2 reflected from the tag 110 can be modulated resulting in a modulated backscatter signal.
The power P2 reflected from the tag 110 is again radiated into free space wherein a small proportion P2′ of this is picked up by the antenna 103 of the interrogator 100. The reflected signal therefore travels into the antenna connection of the interrogator 100 in the backwards direction and can be decoupled and transferred to a receiver 102 of the interrogator 100 where the signal is demodulated in order to retrieve the information transmitted by the tag 110. Alternatively the antenna 103 can be split into a distinct transmit antenna and a separate reception antenna which are spatially separated and thereby eliminating the requirement for a de-coupler in the interrogator 100.
One problem arising with the use of backscatter communication is the appearance of strong so-called inband interferer. In the modulated backscatter system explained above, a part of the emitted radio frequency waveform P1 from the interrogator 100 is feed back into the interrogator receiver 102. Here, it will multiply with a local oscillator (LO) signal and generate an unwanted baseband signal, which is proportional to the power of the leakage from the interrogating waveform, differential delay between the interrogator 100 and the tag 110 and etc. As a result, the performance of the interrogator receiver 102 and the useable range for the modulated backscatter system are degraded.